1167. 'Soils differ greatly in their mechanical as well as in their chemical nature,' observes Professor Solly. 'The same substances constitute a soil possessing very different properties, according as they are in the form of little grains like sand, or in very fine powder. This state of mechanical division is of great importance, for several reasons, and most particular in relation to water. A soil containing a large quantity of alumina is generally known by its stiff tenacious character, and is remarkable for its great retentive power for water; whilst those consisting principally of silica, and more especially those in which it exists in the form of sand, are generally light and porous soils, and far less retentive of water.' (Solly's Chemistry, 2nd edit., p. 139.) 'The silica and alumina in soils, are, of course, almost wholly free and uncombined with any acid, as the former is not a base, and the latter has hardly any affinity for the weaker acids, such as the carbonic. Small quantities of silica are almost always found in soils combined with either soda or potash, forming curious compounds, in which the silica seems to play the part of an acid. Soils never contain more than a very small quantity of these substances; but it is evident that plants, such as grasses, which contain silica, must obtain it from the soil in a soluble form, by gradually absorbing it in combination with alkali, dissolved in water.' (Ibid. p. 140.)
